1. Technical Field
The invention concerns a synchronous digital communications system, a control installation for a network node of a synchronous digital communications system, a network element and a central clock generator for a network node of a synchronous digital communications system.
2. Discussion of Related Art
A synchronous digital communications system is for example an SDH (Synchronous Digital Hierarchy) system or an SONET (Synchronous Optical Network) system. When messages are transmitted via such a communications system, it is essentially necessary that all the network elements (e.g. Add/Drop-Multiplexer, Crossconnects or Line Multiplexer) operate synchronously with each other. ITU recommendation G.803 also states that a network node containing several network elements must be supplied with a reference clock by a single central clock generator of the network node.
A conference article by Dr. M. Wolf, 8th. European Frequency and Time Forum 9., Nov. 3, 1994, TU Munich, pages 166 to 174, describes how the synchronization of the network elements can take place in such a synchronous digital communications system: One network element receives message signals at two inputs, from which a clock is derived and can be used for the synchronization. The accuracy of the clocks in these message signals is provided by a contained quality indicator, hereafter called SSM (Synchronization Status Message, see e.g. ITU-T recommendations G.707, G.708 or G.709). By means of the SSM a selection device in the network element selects one of the clocks as the reference clock and routes it to a central clock generator of the network node. This central clock generator distributes the reference clock to all the network elements of the network node. It also describes that the central clock generator can be an independent unit, or that the clock generator of a network element, preferably that of a crossconnect, can be used as a central clock generator. To prevent the formation of synchronization loops, i.e. where two network elements use each other as mutual reference clock sources, an additional SSM is defined: xe2x80x9cDo Not Use for synchronizationxe2x80x9d, hereafter called DNU. It is transmitted from a network element to all the outputs that are connected to the network element which was selected as the reference clock source.
One problem which is linked to the represented solution is that the network elements of the network node do not know the origin of the selected reference clock, or its accuracy. Nor can the central clock generator provide this information because the clock is selected by the selection device of a network element. As the accuracy of their reference clock, the network elements can only indicate in the SSM the accuracy of the central clock generator during open unsynchronized operation, which as a rule corresponds to the accuracy established in the ITU-T G.812. Also, the SSM DNU cannot be transmitted to the outputs which are connected to the reference clock source used for the synchronization, since this reference clock source is not known to the network elements. This can lead to the formation of synchronization loops.
The object of the invention is to provide a synchronous digital communications system whereby the transmission of a quality indicator (SSM) is made possible as a function of the accuracy of the reference clock source used in the network node. Another object of the invention is to indicate a control installation for a network node of such a communications system. Further objects of the invention are to indicate a network element and a central clock generator for such a communications system.
The object regarding the communications system is fulfilled, according to a first aspect of the invention, by at least one network node, which contains the network elements, and a central clock generator, and with at least one control installation wherein the network elements are connected to the central clock generator for receiving a reference clock from the central clock generator, the central clock generator is connected to at least a part of the network elements for receiving a total of at least two clock signals, each containing a clock and a quality indicator that is indicative of accuracy of the clock, the central clock generator is connected to the control installation for sending messages about its synchronization status to the control installation, and where the control installation is connected to the network elements for sending instructions to the network elements, where the instructions include quality indicators for transmission by the network elements.
Regarding the control installation for a network node of a synchronous digital communications system containing several network elements and a central clock, according to a second aspect of the invention, comprises means for receiving a message about a synchronization status of the central clock generator and means for transmitting instructions to the network elements, where the instructions include quality indicators to be transmitted by the network elements, and which are used to select an external reference clock for network elements that do not belong to the network node.
Regarding the network element with a clock provider which can be tuned to a reference clock, a reference clock input that is connected to the clock provider for receiving the reference clock, and a number of connector units for receiving and sending message signals, where a clock derivation circuit is assigned to at least a part of the connector units, for deriving a clock from the message signal received by the pertinent connector unit, according to a third aspect of the present invention, is characterized by clock signal outputs respectively assigned to one of the connector units with a clock derivation circuit, for sending clock signals containing the clock of the message signal received by the pertinent connector unit, and a quality indicator which is indicative of the accuracy of this clock, and an input for receiving instructions, where the instructions include quality indicators to be transmitted by the network element to the individual connector units.
Regarding the central clock generator for a network node of a synchronous digital communications system, with a clock provider which can be tuned to an external clock, and a reference clock output from which a reference clock can be obtained for network elements of the network node, such is characterized, according to the invention, by a number of clock signal inputs for receiving clock signals, each of which contains a clock and a quality indicator indicative of the accuracy of the clock, a selector unit for selecting the external clock for tuning the clock provider by means of the quality indicators, and an output for sending a message about the synchronization status of the central clock generator to a control installation.
An advantage of the invention is that the formation of synchronization loops is prevented by a synchronous digital communications system according to the invention.